if you have two C5 coupes, both stock, but one has the stock 3.42gearing and the other has 3.90 gearing, how much faster is the 3.90 geared vette going to be?

 

I presume you are speaking of a quarter mile.

I doubt there would be much of a difference as the the 3.90 would probably spin the wheels a little more and would also probably have an extra shift.

 

No extra shift should be needed with 3.90s. I am going to 3.90s over the 3.42s because they will give the car more time in 4th gear to pull. Stock I was shifting into 4th right before the traps so the car did not have much pull time in the gear, and the 3.90s should put me through the traps in the top of 4th.